Ecological details are critical to predicting how invaders will fare

By Eric Sorensen and Jade Boyd

For more than a century and a half, researchers interested in invasive species have looked to Charles Darwin and his “naturalization conundrum” to help explain why some species have an easier time invading new habitats and driving out native species. But a new study suggests that researchers need to be cautious about applying Darwin’s reasoning to 21st-century genomic datasets.

Water hyacinth, which is native to South America, is a problematic invasive species in the rest of the world, where it can take over entire ponds and lakes. CREDIT: E. Jones

“You can’t just jump into making predictions with the data,” said Rice University biologist Emily Jones, lead co-author of the study, which was published online this week in the Proceedings of the National Academy of Sciences. “You have to be careful and know quite a bit about the system before you start trying to predict patterns.”

Darwin’s conundrum relates a niche-versus-competition argument that he laid out in his landmark book, “The Origin of Species.” He reasoned that an invading species would find a more welcoming habitat — and an easier path to invasion — if it was closely related to a native species and could therefore take advantage of an existing niche in the ecosystem. On the other hand, the invader would also face fierce competition from those same closely related native species, and it also would be vulnerable to the natural enemies of those natives.

Emily Jones

Darwin predicted that competition would win out and invading species would have a bigger advantage if they were more distantly related to natives. This focus on species’ relatedness, or phylogeny, has been put to the genetic test in recent years. Scientists have compared the genomes of competing native and invading species to generate measurements of phylogenetic distance. The results have been mixed; in some cases more distantly related invaders fare well and in other cases they do not.

In the new study, Jones, a Huxley Faculty Fellow in Ecology and Evolutionary Biology at Rice, and co-authors Richard Gomulkiewicz of Washington State University and Scott Nuismer of the University of Idaho found that the relatedness of new and native species was not as important as the details of how they went about doing their business.

“We thought we understood how things happened, but maybe they happened for another reason,” said Jones, who started pondering Darwin’s conundrum while a postdoctoral researcher in Gomulkiewicz’s lab.

The Japanese beetle is a invasive pest in the Northeastern U.S., where it threatens some 200 species of plants, including rose bushes, birch trees, grape vines and crape myrtles. CREDIT: E. Jones

“Darwin put out a lot of interesting ideas back in the day, but he didn’t have the means to check them with rigor,” said Gomulkiewicz, a professor in WSU’s School of Biological Sciences. “That’s what we did with our mathematical model, and we found that Darwin’s logic on this issue doesn’t quite pan out.”

The model they’ve developed in analyzing Darwin’s conundrum could lead to a new way of gauging the potential of invasive species, a major ecological and economic concern as plants and animals have spread into new habitats around the planet.

Jones and her colleagues found that species’ phenotypes — characteristics that emerge as a plant’s or an animal’s genes interact with the environment — were a more important predictor of invasion success. They found that genetic relationships alone are a weak predictor of invasion success.

To be sure, Jones said, researchers will want to see what species an invader is related to and what interactions that species has that are important for understanding its survival. However, they will also have to consider the different mechanisms through which phenotypes govern the outcomes of interactions between species.

“We know that different mechanisms operate in all sorts of species’ interactions, but there isn’t a great deal that’s known yet about which are more important or which are more common, and that makes it harder to generalize,” Jones said. “That’s not saying that you can’t use phylogenetic distance as a predictor. You just have to know more about the biology and the natural history of the species.”

About Jade Boyd

Jade Boyd is science editor and associate director of news and media relations in Rice University's Office of Public Affairs.

2 Comments

Jenn Drummond

Posted December 9, 2013 at 6:17 PM

When Darwin said “closely related”, was he necessarily speaking genetically/phylogenetically? Or could *he* have meant phenotypically? Given what we know about mechanisms of speciation these days, isn’t it actually likely that two very closely related species in the same geographical area will be pretty divergent in their resource usage? The closest match niche-wise might well be some critter that’s only “related” by convergence! Clearly I need to go read those bits in the *Origin*. Thanks for a good article!

Thanks for your questions! Darwin did mean related in a phylogenetic sense, as he compared introduced species from genera that were and were not already represented in the introduced range. Certainly I would agree that we would expect sister species that had been diverging sympatrically (in the same geographic area) to differ in their resource use. However, the question here is about what happens when species diverge allopatrically (in different geographical areas) for some time and then come into contact again. In this case, we shouldn’t necessarily expect phenotypic divergence to be strongly correlated with phylogenetic distance.

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